Organic solvent based epoxy resins for coating objects to improve corrosion resistance and moisture resistance is well-known. Organic solvent based epoxy resins have, for example, been used to coat cans to prevent corrosion of the can from the atmosphere or contents of the can.
More recently due to environmental concerns, water borne epoxy resins have been developed. In general, water borne epoxy resins can be split into two categories: (I) those using liquid epoxy resins (i.e., those having less than about 500 molecular weight) and (II) those using solid epoxy resins (i.e., those having a molecular weight of greater than about 500 molecular weight).
However, most water borne epoxy resins and, in particular, those containing solid epoxy resins have utilized some organic solvent to disperse and enhance coalescence of the dispersed particles upon being coated onto a substrate. In the absence of a solvent, the epoxy resin particles generally fail to coalesce sufficiently to provide a coating suitable, for example, to prevent corrosion of a metal can.
Because water catalyzes the epoxy resin-curing agent reaction, water based epoxy resin coatings have tended to have a short pot life (i.e., limited time to apply the coating composition before it gels in the pot after mixing in the curing agent). In particular, coating compositions containing liquid epoxy resins and aliphatic amine curing agents, such as polyethylene amine, have suffered from very short pot life. In general, to somewhat compensate for this problem, the curing agent or hardner is usually made by reacting, for example, the polyethylene amine with (i) fatty acids, (ii) dimer acids or (iii) epoxy resin, followed by a treatment to reduce the primary amine content. Because of the strong catalytic effect of water on the amine epoxy reaction, the above modification to the amine is almost always employed. Even with the above modification, the pot life is generally less than 3 hours.
To aid in emulsification of the epoxy resin and pot life of liquid epoxy resins, these resins generally have required the grafting of dispersing moieties (e.g., alkylene oxides chains) to epoxy resins and, in particular, type (II) resins. The incorporation of dispersing moieties increases the cost significantly due to the complicated chemistry and processing to form a stable dispersion or emulsion.
Due to decreased stability of the water based epoxy emulsion or dispersion, inorganic pigments useful in improving the aesthetics or corrosion of the metal substrate or wear of the coating generally have not been used in these compositions. This instability is especially evident when the curing agent, such as an amine, is combined with the water-based epoxy resin composition. The curing agent, such as an amine, tend to be basic and, thus, can cause the emulsification or dispersion of the epoxy and/or pigment to coagulate.
Thus, it would be desirable to provide a water-borne epoxy resin coating composition that avoids one or more of the aforementioned problems with these coating compositions.